RELATIONSHIPS OF ECHINODERMA. 279 
the peculiar water-vascular system, may all disappear ; it is conceivable 
that further modification of the same kind might eliminate all the dis- 
tinctively Echinoderm characters, and produce an organism whose 
systematic position would be very difficult to determine. This is 
important, because, as we have already seen, there are many ‘‘ worm- 
like” types of whose affinities we know nothing. That some of these 
are related to Echinoderms has been often suggested. 
It is conceivable that Holothurians of the worm-like Syxag¢a type 
are nearest the primitive stock of Echinoderma. But there are strong 
arguments in favour of the view that the free forms, the Eleutherozoa, 
have been derived from attached Pelmatozoic ancestors. The extinct 
Edrioasteroidea are in some ways intermediate between the Cystidea 
and the Eleutherozoa. 
